1. Field of the Invention
The present invention relates to an image processing method and apparatus, a threshold value matrix creating method, an image forming apparatus and a sub-matrix creating method and program, and more particularly, to image processing technology suitable for a halftoning process whereby image defects caused by ink ejection failures in an inkjet printer are made to be inconspicuous.
2. Description of the Related Art
In an inkjet printer, for a variety of reasons, a situation may occur in which it becomes impossible to eject ink from a nozzle. For example, when a particular nozzle in a group of nozzles fails to eject or malfunctions, the dots that should have originally been deposited by the nozzle are missing, which results in unintended flaws in the form of lines (streaking) in the recorded image on the recording medium, and this streaking is extremely conspicuous.
In particular, in the case of a device composition which completes printing by means of a single sub-scanning operation (a single pass method), using a line-type recording head in which a plurality of nozzle are arranged, it is difficult to cover the droplet deposition position of a malfunctioning nozzle suffering an ejection failure, by means of another nozzle (in other words, a so-called “shingling” operation), and streaking non-uniformity due to malfunctioning nozzles suffering an ejection failure is highly notable, leading to serious deterioration in image quality. Therefore, a method has been proposed by which, if any of the nozzles is suffering an ejection failure, deterioration in image quality caused thereby is made to be inconspicuous.
Japanese Patent Application Publication No. 2004-202795 discloses technology for an inkjet printer in which a normal dithering matrix and a dithering matrix corresponding to ejection failure are prepared in order to that blanking out caused by ejection failure does not become conspicuous, the dithering matrices being used selectively on the basis of ejection failure position information.
Furthermore, Japanese Patent Application Publication No. 2004-202927 discloses technology which changes the combination of recording dots in such a manner that ejection is performed by other recording elements, instead of recording dots deposited by malfunctioning recording elements suffering an ejection failure.
However, the technology which changes the nozzles to a combination which does not include malfunctioning nozzles as disclosed in Japanese Patent Application Publication No. 2004-202795 has a drawback in that, if there are a plurality of malfunctioning nozzles within a dithering matrix, then it becomes necessary to provide dithering matrices in accordance with the number of combinations of the nozzles, and the storage volume of all of the dithering matrix data becomes extremely large. Furthermore, technology which changes the combination of dots as disclosed in Japanese Patent Application Publication No. 2004-202927 is not compatible with cases where dithering matrices having a size of 2×2 pixels or above are used, since the number of combinations involved is enormous.
In general, dot image of high quality is obtained if the input image is quantized by using a threshold value matrix of a large size, typically, a blue noise mask (for example, 256 (columns)×256 (rows) of pixels, or 512 (columns)×512 (rows) of pixels, or the like). If the technology disclosed in Japanese Patent Application Publication No. 2004-202795 is used with the object of reducing streak-shaped artifacts caused by ejection failures when using a blue noise mask of this kind (i.e., technology which previously prepares a plurality of threshold value matrices corresponding to malfunctioning nozzles and switches the matrices in accordance with the ejection failure positions), then supposing that this technology is applied to threshold value matrices of a large size (for example, M=N×N), the number of threshold value matrices prepared for use in switching to correspond to ejection failures will be of the order of M×N (where M is the matrix size and N corresponds to the ejection failure positions contained in the threshold value matrix), in other words, N×N×N (N3), and consequently, an enormous storage volume is required for all of the threshold value matrices corresponding to ejection failures.
In the case of one ejection failure, the number of matrices is of the order of N3 as described above, and supposing that two or more ejection failures occur within one matrix, then it can be seen readily that matrices of the order of N4 will be required. If the threshold value matrix size is reduced, then the storage volume is reduced accordingly, but in this case, it is not possible to use a blue noise mask, and hence high image quality can not be obtained.